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I have recently been working on an Andrew 4.6m Ku-Band antenna fitted with a 4-port feed making it ready to carry traffic. During the testing I carried out Azimuth and Elevation cuts which passed the 29-25log Theta masks, G/T measurements and Antenna Gain measurements. The issue that I came across was when carrying out the cross polarisation check with SES. The antenna failed the cross pol check in the V-Pol as it could only achieve a separation of 25dB. The cross pol on the H-Pol was 30dB. The 4 port feed was rotated by 90 degrees as it is necessary to transmit on the V-Pol. I fully expected the issue to rotate with the feed however, on carrying out the test again the V-Pol failed and the H-Pol passed with the same figures. Further checks were then carried out on the antenna and system including visual inspections of the feedhorn assembly, OMT and a physical check of the sub-reflector position. The sub-reflector was re-focused and went back to the same position as it started from. Again the antenna was cross poled with SES and again failed on the V-Pol. Just to confirm that the issue was not with SES I carried out a cross pol check with Eutelsat and got the same results. Andrew Technical Support have been contacted in the USA and they asked to carry out all the checks that I have previously described. They are also stumped by this one and have gone away to have another think about it. I have never come across an issue like this before and would like to throw the topic open to the forum to see if I can get any feedback as to what could be causing the problem.
Best Regards,
Graeme

A quick update in case anyone is reading this one - Andrew are stumped by this one and have stated they have never seen this type of issue before. We are in the process of ordering a new OMT, Feed and Sub-reflector. If none of these items solve the issue then it can only be the main reflector which should show up on the cuts. Unfortunately it is a 10 to 12 week lead time to order the items, so please don't hold your breath waiting to see how this one ends.

We nailed it and it was not a good discovery. The feed itself was misaligned and has been since installation in around 2002. This has now been rectified and we are achieving a cross-polarisation separation of >30dB. I am not going to say any more on an open forum other than investigation are continuing to how this happened in the first place.